The long-term objective of this research is to better understand certain individual and group differences in audition. One of the group differences of interest is sex differences. Many auditory measures-some physiological and some behavioral (psychophysical)-exhibit large sex differences. Among the former are otoacoustic emissions (OAEs), which are sounds produced by the inner ear. Because sex differences exist in the OAEs of infants and children as well as in adults, they must originate, at least in part, during prenatal development. Because many other sex differences in body, brain, and behavior are known to be produced by differential exposure to androgens during prenatal development, the implication is that the sex differences in OAEs also are produced by differential androgen exposure. One goal of the planned research is to determine exactly what the large sex and individual differences in OAEs mean for performance on various psychophysical tasks. For example, what psychophysical tasks benefit from strong OAEs? Along with young-adult listeners having normal hearing, monozygotic (MZ) female twins and opposite-sex dizygotic (OSDZ) female twins will be tested because these special populations have been shown to have atypically strong and weak OAEs, respectively. This work is relevant to public health because knowledge of the relationships between physiological and psychophysical measures of hearing will reveal what aspects of auditory perception will be impaired in a person (say, an infant) having weak OAEs, and that information should eventually be beneficial during intervention. Also, this work is in accord with the primary goal of modern neuroscience which is to learn more about the relationships between physiological measures and behavior. These studies also will provide additional evidence on the implication that the inner ear can be affected by exposure to androgens early in development. In one of the planned studies, auditory evoked potentials (AEPs)-another physiological measure exhibiting sex differences-will be measured in children having attention-deficit/hyperactivity disorder (ADHD). We have already shown that some of these children have OAEs suggesting that they were exposed to high levels of androgens at some point early in development, and it is important to know if AEP measures confirm this finding. If so, theory, diagnosis, treatment, and nosology for ADHD could be affected.